More than 500 Fosamax users from 45 states contend the company failed to warn them or their doctors of the danger, despite early evidence suggesting the increase potential for spontaneous bone breaks without any previous stress.

Merck, which does not dispute the risk and has included a warning with prescriptions since 2010, argued it cannot be held liable for damages in state courts because the Food and Drug Administration in 2009 rejected a proposed warning to patients.

"When the FDA exercises this authority, it makes careful judgments about what warnings should appear on a drug's label for the safety of consumers," Justice Stephen Breyer wrote in the court's opinion.

"For that reason, we have previously held that 'clear evidence' that the FDA would not have approved a change to the drug's label preempts a claim, grounded in state law, that a drug manufacturer failed to warn consumers of the change-related risks associated with using the drug," he wrote.

The justices clarified that it should be left to judges, not a jury, to decide the preemption question in drug-harm suits, as occurred in this case. But they left the door open for patients to challenge drug companies on the facts surrounding information provided to the FDA -- up front -- to be used in a labeling decision.

"This opinion protects access to justice for injured patients," said David Frederick, a lawyer for the Fosamax users who sued Merck.

Frederick noted that the court did not dismiss plaintiffs claims outright on substance, ruling narrowly on the technical question of how preemption should be determined.

Patient advocates have said a decision siding with Merck on the merits would embolden drug manufacturers to provide insufficient and misleading information to FDA, in effect insulating themselves from potential legal liability. "It's the pharmaceutical company's job to write the drug label," said Medshadow's Sue Robotti.

Manufacturers are required by law to inform patients of potential adverse reactions to their drugs as soon as reasonable evidence exists. But the FDA has ultimate authority to approve or reject the wording that appears on drug labels.

In a 2008 application to the FDA, Merck proposed revising the warning language for Fosamax, describing a heightened risk of "stress fractures."

One year later, the FDA rejected that draft language, saying the warning was "not warranted and is not adequately supported by the available literature" and asked for revised language.

Merck said the FDA's conclusion, based on available evidence at the time, means the company cannot be held liable for failing to warn consumers as required under state law because the federal government wouldn't allow it.

In a statement, the company said it welcomed the court's decision and "remains fully committed to defending these cases going forward and will continue to present evidence that it acted appropriately at all times in regard to the potential risk of atypical femur fractures."